Processes Prior to Outbursts of Vulcanian Eruption at Showa Crater of Sakurajima Volcano(＜Special Section＞Sakurajima Special Issue)

Showa crater of Sakurajima volcano became active in June 2006 after 58 years of quiescence. From multiparametric geophysical observations, we have identified the processes that typically occur prior to an explosive eruption at the crater. A few hours prior to the onset of an eruption, magma starts to migrate and accumulates at a depth of about 1 km. This accumulation of magma can be clearly observed in strain change records as an inflation process. Several tens of minutes prior to an eruption, the SO2 gas emission rate gradually decreases, indicating that a sealing process is taking place in the crater bottom as the eruption nears. During the same time period, the volcanoʼs inflation rate starts to accelerate due to the formation of a plug above the conduit that prevents the gas from escaping, with the result that a gas pocket forms beneath the crater. In nighttime events, a volcanic glow is also seen, which weakens and then disappears. A few minutes prior to an eruption, a small tremor starts to occur. Its amplitude grows as the strain changes from inflation to deflation as the stored gas is released through new fractures within the plug that had been confining the gas pocket, leading to a minor depressurization in the conduit. Then, an expansion process starts, that could explain seismically the first motion of an explosion earthquake. This is probably when the effect of depressurization downward from the crater bottom reaches the magma head and a sudden magma expansion with degassing starts. After a short period (about half a second), this expanding magma rises and pushes the gas pocket upward, leading to a swelling of the crater ground along with the radiation of the preceding phase of infrasound waves, and then a breakup occurs. After the plug fails due to deformation, the accumulated gasses and expanding magma are ejected together from the crater as the surface eruption phenomena starts.